8. Introducing Spring Cloud Stream

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8. Introducing Spring Cloud Stream
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8. Introducing Spring Cloud Stream

The Spring Cloud Stream project allows a user to develop and run messaging microservices using Spring Integration and run them locally, or in the cloud, or even on Spring XD. Just add @EnableBinding and run your app as a Spring Boot app (single application context). You just need to connect to the physical broker for the bindings, which is automatic if the relevant binder implementation is available on the classpath. The sample uses Redis.

Here’s a sample source module (output channel only):

@SpringBootApplication
@ComponentScan(basePackageClasses=TimerSource.class)
public class ModuleApplication {
  public static void main(String[] args) {
    SpringApplication.run(ModuleApplication.class, args);
@Configuration
@EnableBinding(Source.class)
public class TimerSource {
  @Value("${format}")
  private String format;
  @Bean
  @InboundChannelAdapter(value = Source.OUTPUT, poller = @Poller(fixedDelay = "${fixedDelay}", maxMessagesPerPoll = "1"))
  public MessageSource<String> timerMessageSource() {
    return () -> new GenericMessage<>(new SimpleDateFormat(format).format(new Date()));
}

@EnableBinding is parameterized by an interface (in this case Source ) which declares input and output channels. Source , Sink and Processor are provided off the shelf, but you can define others. Here’s the definition of Source :

public interface Source {
  @Output("output")
  MessageChannel output();
}

The @Output annotation is used to identify output channels (messages leaving the module) and @Input is used to identify input channels (messages entering the module). It is optionally parameterized by a channel name - if the name is not provided the method name is used instead. An implementation of the interface is created for you and can be used in the application context by autowiring it, e.g. into a test case:

@RunWith(SpringJUnit4ClassRunner.class)
@SpringApplicationConfiguration(classes = ModuleApplication.class)
@WebAppConfiguration
@DirtiesContext
public class ModuleApplicationTests {
	@Autowired
	private Source source
	@Test